HISTORY OF PHYSICAL ASTRONOMY.

385

side where the sun had gone in, then below, and then on the rightside; the light coming out at each place successively, like little beamsfrom the moon’s edge. Such are the terms in which the variation ofthe appearance of the ring as viewed from the Superga, is described*.Sig. Piola states, that at St. Angelo, near Lodi, the same remark was madeby every person who witnessed the eclipse j. It has been already mentionedthat a similar fact was uoticed, with respect to the appearance of thering, during the total eclipses of 1784 and 1733.

The question relative to the physical came of the luminous ring hasgiven rise to much speculation. Some persons have supposed it to deriveits origin from the moon, while others again have maintained that it ispurely a solar phenomenon. A brief historical statement of the differenthypotheses that have been advanced in connexion with this subject maynot, perhaps, prove uninteresting to the reader.

Various explanations of the luminous ring have been advanced in ac-cordance with the supposition of its being an appendage of the moon.The earliest is that which ascribes it to the influence of a lunar atmo-sphere. It was first suggested by Kepler, as a probable mode of ac-counting for the phenomenon. He conjectured [that the rays oflight proceeding from the sun to the earth, might be refracted in passingthrough the moon’s atmosphere, and might thereby occasion an appearanceresembling the luminous ring This view of the origin of the pheno-menon was regarded with favour by Halley, although at the same timehe admitted that it could not be considered as fully established. If thering was due to the existence of a lunar atmosphere, it ought invariablyto appear concentric with the moon. On the other hand, if it arosefrom the presence of an atmosphere about the sun, it could only surroundthe moon equably at the instant when the centres of the two bodies werein conjunction. In the latter case, it is manifest that the ring ought toappear broadest first at the point where the sun had just disappearedbehind the moon’s limb, and afterwards at the opposite point where hewas about to emerge. It is easy to see, therefore, that observations onthe position of the ring during an eclipse, if executed with precision, mightafford a valuable criterion for deciding the question, whether the ring was anappendage of the sun or moon. All such observations have, however, beenhitherto so vague and contradictory, that no reliable conclusion can bededuced from them. The ring fades off so imperceptibly from the moon’slimb, that the establishment, beyond all doubt, of a variation in its breadthduring the passage of the moon across the solar disk, if even such a varia-tion existed, would seem to be impracticable. In some instances it hasbeen affirmed that the ring appeared concentric with the moon, while inothers it has been maintained with equal confidence, that it was concentricwith the sun. The observations in general are decidedly more favourableto the supposition of the ring being concentric with the sun, than to itsholding a similar relation with respect to the moon§. But indeed it is

* Mem. Ast. Soc., vol. xv., p. 16.

+ Gior. dell’ 1st. del Lomb., tom. iv., p. 606.

f Ad Vitellionem Paralipomena, p. 302; Epitome Astronomise, p. 893.

§ Halley, while inclined to suppose that the luminous ring which appeared during thetotal eclipse of 1715, arose from the presence of an atmosphere about the moon, did notdeny at the same time, that some persons found the ring to increase in breadth as theemersion approached. “ This circumstance,” says he, “ together with the contrary sen-timents of those whose judgment I shall always revere, makes me less confident” {Phil.Trans. 1715, p. 249). It is not improbable that Halley here alludes to the illustriousNewton, with whom he lived on terms of intimate friendship.